Elsevier

Materials & Design

Volume 57, May 2014, Pages 632-637
Materials & Design

Friction welding of Al–12Si parts produced by selective laser melting

https://doi.org/10.1016/j.matdes.2014.01.026Get rights and content

Highlights

  • Friction welding used to join Al–12Si parts produced by selective laser melting.

  • Welding induces significant microstructural variations in the welded zone.

  • Reduced hardness is observed in the weld zone compared to the base metal.

  • Yield strength of welded sample lower than in as-prepared condition.

  • Welding induces remarkable increase of ductility.

Abstract

Al–12Si samples produced by selective laser melting (SLM) are welded using solid-state friction welding. The weld metal shows the presence of texture with excess Si diffusing out from the Al matrix. Microstructural investigations reveal a pronounced change in the shape and size of the Si phase in the weld metal compared to the base material, with the formation of extremely fine particles uniformly distributed in the Al matrix. This variation in the microstructure is expected to have significant changes in the mechanical properties of the welded material. The hardness measurements reveal a drop of hardness in the weld zone with respect to the base metal. Similarly, the room temperature tensile tests show a significant improvement of ductility in the welded SLM samples. However, the yield and the ultimate strength show only a marginal drop in the welded samples compared to the as-prepared SLM specimens. The present work demonstrates that solid-state friction welding not only permits to successfully join materials produced by SLM, but also helps to significantly improve their ductility.

Introduction

Selective laser melting (SLM) is a three dimensional additive manufacturing process which produces metal parts with high degree of geometrical freedom from 3D computer models, such as computer-aided design CAD data [1], [2], [3]. The SLM part is generated layer by layer through the selective melting of the powder as dictated by the CAD model. Due to the layer by layer processing, SLM allows for the production of objects with intricate shapes and complex geometries that would be extremely difficult or impossible to fabricate through conventional subtractive manufacturing techniques [4]. A major drawback for the wide application of SLM as an industrial processing route is the limited size of the products. This is a direct consequence of the limited dimensions of the available building chambers, which allow for the production of samples with volumes of about 0.02 m3 [5]. A possible way to overcome this problem would be the use of the welding processes to join the small SLM objects to form parts with no dimensional limitations.

In this work, we have analyzed this possibility by welding Al–12Si parts produced by SLM. The yield strength of the Al–12Si alloy processed by SLM is four times higher than yield strength of a conventionally cast Al–12Si alloy [3]. Such high-strength alloys cannot accommodate the stresses that arise during the fusion welding processes, which may lead to the formation of cracks [6]. To avoid such solidification related problems, solid-state welding can be utilized. This method does not present issues related to solidification cracking, liquation cracking, segregation and formation of brittle eutectics/intermetallics [6], [7]. In addition, solid-state welding results in fine-grained microstructures with superior mechanical properties compared to the conventional fusion weld processes and in a narrow heat affected zone and low residual stresses in the weldment [6], [8], [9].

Among the solid-state joining processes, friction welding (FW) has drawn considerable attention due to economic considerations and high productivity [5], [9]. In this process, heat is generated by the conversion of mechanical energy into thermal energy at the interfaces of the parts, rotated under pressure. Friction time and pressure, upset time and pressure, and rotation speed are the main parameters that govern the FW process. Compared with other welding techniques, friction welding displays advantages such as high materials saving; short joining time and possibility of making dissimilar joints [9], [10], [11], [12], [13], [14], [15].

The analysis of the effects of welding on the microstructure and mechanical properties of the joined SLM parts is of particular interest for the possible implementation of this type of material into a conventional industrial processing line, such as automated or robot welding for high production applications. Accordingly, in this work structural and microstructural characterizations are performed on the welded parts. The mechanical properties of the welded samples are analyzed through hardness measurements and room temperature tensile tests followed by detailed fracture surface analysis. Factors leading to failure during the tensile tests are discussed and compared with the corresponding welded parts produced by casting.

Section snippets

Experimental details and sample preparation

Cylindrical rods of 12 mm diameter and 60 mm length were produced on a Al substrate plate by SLM from spherical gas-atomized powder with nominal composition Al–12Si (wt.%). SLM processing was carried out under high purity argon at room temperature using an SLM 250 HL device (SLM Solutions) equipped with an Yb–YAG laser. The parameters used for preparing the samples are: scanning speed 1455 mm/s for the volume and 1939 mm/s for the contour, power 320 W both for the volume and contour, layer thickness

Structural analysis

The production of the Al–12Si samples by SLM has been reported in detail elsewhere [3] and only the contents related with the present manuscript are discussed hereafter. Fig. 1(a) shows a typical image of a friction welded Al–12Si joint with symmetrical and smooth flash at the joint, indicating adequate heat generation, plastic deformation and expulsion of oxide scales and other contaminants during the welding process [17], [18].

The XRD patterns of the base metal for the samples prepared by

Conclusions

In this work, the effect of welding on the microstructure and mechanical properties of Al–12Si parts produced by SLM has been analyzed in detail. Friction welding was selected to join the SLM parts in order to avoid solidification related problems, such as cracking, which may result from the high-strength of the Al–12Si alloy processed by SLM. Phase analysis reveals that the amount of free Si significantly increases in the SLM-weld zone compared with the base metal. This is accompanied by the

Acknowledgement

K.G. Prashanth would like to thank Dr. G. Phanikumar, Indian Institute of Technology Madras (IITM), Chennai for his support to carry out the friction welding experiments at IITM, India.

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